Re: cheap way to test "doorknob" capacitors? (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 12 Nov 2004 09:13:41 -0700
From: Gomez Addams <gomez@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: cheap way to test "doorknob" capacitors? (fwd)


On Nov 12, 2004, at 8:38 AM, High Voltage list wrote:

> Original poster: <sroys@xxxxxxxxxxxxxxxxx>
>
>
>
> ---------- Forwarded message ----------
> Date: Thu, 11 Nov 2004 14:40:18 -0600
> From: Shaun Epp <scepp@xxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Re: cheap way to test "doorknob" capacitors? (fwd)
>
>
> ----- Original Message -----
> From: "High Voltage list" <hvlist@xxxxxxxxxx>
> To: "hvlist" <hvlist@xxxxxxxxxxxxxxxxx>
> Sent: Thursday, November 11, 2004 9:27 AM
> Subject: Re: cheap way to test "doorknob" capacitors? (fwd)
>
>
>> Original poster: <sroys@xxxxxxxxxxxxxxxxx>
>>
>>
>>
>> ---------- Forwarded message ----------
>> Date: Thu, 11 Nov 2004 07:09:50 -0700
>> From: Gomez Addams <gomez@xxxxxxxxxxxx>
>> To: High Voltage list <hvlist@xxxxxxxxxx>
>> Subject: Re: cheap way to test "doorknob" capacitors? (fwd)
>>
>>
>> On Nov 10, 2004, at 7:14 PM, High Voltage list wrote:
>>
>>> Original poster: <sroys@xxxxxxxxxxxxxxxxx>
>>>
>>>
>>>
>>> ---------- Forwarded message ----------
>>> Date: Wed, 10 Nov 2004 19:31:13 -0600
>>> From: Shaun Epp <scepp@xxxxxxx>
>>> To: High Voltage list <hvlist@xxxxxxxxxx>
>>> Subject: Re: cheap way to test "doorknob" capacitors? (fwd)
>>>
>>> I read that the capacitance of door knob caps are a function of
>>> voltage.
>>> The capacitance is higher at high voltages.
>>
>> Wouldn't be a very good capacitor if that were true.  It's news to me.
>> Got a reference?
>>
>
> Unfortunatly I don't have a reference, but I remember the explanation 
> saying
> it had to do with the electric field moving further outwards from the 
> plates
> icreasing the effective capacitance as the voltage across the plates
> increases.
>
> Now that I think of it, this doesn't sound right.  The electric field 
> will
> always be present around the plates with voltage applied, more voltage 
> would
> cause a stronger field.  Now I'm not sure!.  The only way this could 
> happen
> is if the dielectric was a semiconductor and the voltage cause partial
> conduction of the dielectric effectively moving the plates closer 
> together.
> Which doesn't sound right either.

  I have heard of that phenomenon, but I thought it was something that 
occurred only in caps where the plates were free to move a bit, such as 
oil-filled paper dielectric DC filter caps.  One would think it 
wouldn't happen much in ceramics.  On the very other hand, I just 
remembered that barium titanate ceramic is a piezoelectric material!  
Which would definitely support your assertion!  I guess getting a good 
night's sleep and a cup of coffee helped jog my memory.  :)

  - B(G)L,
	Denver


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